We aren't the only ones who want Gothwan to reveal the location of Karzov and the Crown. A gang of wererat thieves, known as the Dead Rats, caught wind of his involvement and kidnapped him. The Nashers abandoned Gothwan to his fate, so if you can rescue him he will likely tell us everything he knows.
The Dead Rats lair in the sewers. Here, I'll mark a known entry on your map, but be careful. Gothwan is no doubt being kept deep within their nest.
Yes, I helped steal the Crown of Neverwinter! I was a loyal Nasher, but after Karzov left me to die down here, I don't owe him anything. I'll tell you what I know about where Karzov is now, and also where he's stashed the Crown.
Hey, if you see that treacherous bastard, remind him that loyalty works both ways - preferably while you're twisting a blade in his guts!